1. Field of the Invention
The present invention pertains to acquisition of seismic data. More particularly, the present invention is related to techniques for acquiring seismic data while minimizing effects due to unwanted out-of-plane energy signals arriving at the receivers, and is especially applicable to marine seismic acquisition. Specifically, the present invention relates to receiver placement in seismic exploration.
2. Description of Prior Art
Seismic exploration is carried out by generating acoustic or elastic waves by one or more sources which direct the wave fronts into the earth's subsurface. Wave fields reflected by subsurface structures, or horizons, are received at the surface by detectors, or receivers, such as geophones. Electrical cables connect the receivers to a monitor which records the electrical signals produced by the receivers in response to the detected acoustic or elastic waves. For seismic exploration conducted through a body of water, the receivers are hydrophones positioned along a cable called a streamer.
Signals from an array of receivers comprising a single line may be utilized to provide information about subsurface structures generally lying along a vertical plane, that is, the selected, or desired, vertical plane defined by the line of the source and the receivers. With such information, a 2-D vertical, seismic section of the subsurface may be produced, in pictorial form, for example. A receiver and source arrangement comprising a multiplicity of generally parallel, and relatively closely-spaced lines of receivers and sources provides data which may be utilized to produce a 3-D representation of subsurface structures. In general, multiple parallel lines of sources and receivers are used for data acquisition. In marine seismic acquisition, multiple parallel streamers may be towed by a single boat that traverses multiple parallel paths to comprise the total 3-D survey.
In 2-D marine seismic acquisition there is frequently a problem with energy from out of the selected plane arriving at the receivers along with the in-plane energy. The out-of-plane energy reduces the accuracy and reliability of the accumulated data as representing the underground structures. An additional problem, unrelated to the out-of-plane energy, is that the signal bandwidth is reduced because of ghosting related to the depth of the marine receivers as signals are reflected from the water surface to the receivers.
A currently available technique for dealing with the out-of-plane noise problem involves the use of multiple, parallel streamers. Data from the streamers are added in the crossline sense to attenuate the out-of-plane arrivals. However, the spacing of the streamers is not optimized, but is more determined by rules of thumb. Since the streamer spacing affects the crossline attenuation, that attenuation is not maximized. Further, the streamer depth, which is generally held constant for all streamers, produces bandwidth loss due to ghosting.
Some of these effects may be further appreciated by reference to FIG. 1 which shows a computer model simulation of the amplitude response of the crossline addition of the output of four receivers in four parallel streamers fifty meters apart and at a constant depth of nine meters. The amplitude of the summed received signals, indicated in decibels according to the color scale presented at the left of the drawing, is plotted as a function of frequency from zero Hz to 125 Hz along the ordinate, and of crossline dip from -90.degree. to +90.degree. along the abscissa. The in-plane data is along the 0.degree. ordinate. The parameters on which the frequency spectrum of FIG. 1 is based are fairly typical for a four-streamer array, and it is instructive to observe several features of the amplitude response.
First, at low frequencies the receiver array passes most energy regardless of dip as indicated by the broad dip angle band A. Second, at high frequencies signals for a relatively narrow range of dip are passed, that is, signals are passed only for in-plane arrivals, or for nearly in-plane arrivals within the band B. Third, for certain combinations of frequency and dip the arrivals are passed unattenuated because the out-of-plane arrivals are aliasing as in-plane arrivals. These signals combine in-phase to produce the high amplitude, curved lobes C, for example. Fourth, the amplitudes are not uniform, but rather exhibit peaks and nulls related to the ghosting. h-plane peaks D are centered at 42 Hz and at 125 Hz, and in-plane nulls appear at 0 Hz and at 83 Hz, indicated at E. Fifth, ghosting causes phase distortion which fluctuates back and forth between +90.degree. and -90.degree. as may be appreciated by reference to FIG. 2 wherein is shown the phase response for the array whose amplitude spectrum is illustrated in FIG. 1. As shown in FIG. 2, abrupt 180.degree. phase changes appear with changing frequency and dip at various locations throughout the phase spectrum. The parabolic ghosting notch, which produces the in-plane null E in FIG. 1, is evident in both FIGS. 1 and 2.
Any of the aforementioned five features of crossline summation may present a serious problem to data acquisition depending on the frequencies of interest and the nature of the crossline noise.
Another technique currently available for dealing with out-of-plane arrivals involves collecting 3-D data and using migration to move the out-of-plane energy to its proper plane. While this is an effective approach it has the disadvantage of greatly increased expense.
Another approach to out-of-plane energy problems addresses the loss of signal which is slightly out-of-plane, especially at high frequencies, as noted above. In this technique signals are scanned over a selected, relatively small range of acceptable angles. As a result signal strength can be improved, particularly at high frequencies where signals are normally attenuated due to the sharpness of the array response. However, this technique requires signals that are strong enough to be detected, and only one at a time can be handled. Also, this technique does not optimize to reduce coherent noises.
Crossline summation of multistreamer receivers can, in general, attenuate out-of-plane energy and also attenuate random noise. It would be advantageous and desirable to provide a technique for utilizing crossline summation in a manner to avoid the aforementioned shortcomings of that approach. The present invention provides such a method wherein the spacing of multiple streamers, for example, is optimized to avoid or minimize these drawbacks.